Computer system, host-bus-adaptor control method, and program thereof

ABSTRACT

A computer system includes: a first HBA that constitutes an operational path and on which a first PCI/PCIe-bus expansion card having a PCI PM function, which is a power saving function, is mounted; and a second HBA that constitutes a standby path and on which a second PCI/PCIe-bus expansion card having the PCI PM function is mounted; and a control section that controls the first HBA and the second HBA. The control section first uses the operational path by setting the first PCI/PCIe-bus expansion card to an active state. When a failure occurs in the operational path, the control section performs control such that communication recovery processing for the operational path is applied to the first HBA and further the second PCI/PCIe-bus expansion card of the second HBA is changed from a power saving state to which the second PCI/PCIe-bus expansion card has been first set, to an active state. When the communication recovery processing ends unsuccessfully, the control section performs communication with the I/O device by using the second PCI/PCIe-bus expansion card of the second HBA.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2011-140897 filed on Jun. 24, 2011, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a computer system, a host-bus-adaptorcontrol method, and a program thereof. In particular, the presentinvention relates to, in a computer system in which a host computer andan I/O device are connected via redundant paths by using host busadaptors on which PCI/PCIe-bus expansion cards are mounted, power savingcontrol for the PCI/PCIe-bus expansion card mounted on the host busadaptor that is provided in a standby path.

BACKGROUND OF THE INVENTION

In mission-critical computer systems, multiplexed (redundant) I/O pathsto an I/O device, such as a storage system, are used in order to improvethe reliability. Furthermore, among recent computer systems, a system inwhich a host computer and an I/O device are connected by using aPCI/PCIe-bus expansion card has been put into practical use. Regardingthe multiplexing of an I/O path, for example, Japanese Unexamined PatentApplication Publication Nos. 2004-185093 and 2007-265243 disclosetechnologies in which multiplexing an I/O path to a storage systemimproves the input-output performance, and, when a failure occurs, anormal logical path is selected to access a logical unit in the storagesystem.

Furthermore, in recent years, the speed of host bus adaptors(hereinafter, referred to as HBAs) in I/O paths in computer systems hasbeen increased. This increase in speed can be realized by making the I/Opaths redundant, but the redundant I/O paths may implement a higherperformance than the required input-output performance. In such a case,it is desirable from the standpoint of power saving that only thedevices in the minimum necessary I/O path (operational path) be operatedin order to attain the required input-output performance, and thedevices in the spare I/O path (standby path) be stopped.

However, to activate a device that has been stopped, for example, a hostbus adaptor, a start time of several seconds to several tens of secondsis required to switch from the stopped state to a state in whichcommunication can be performed. Thus, in the mission-critical computersystems, the technique in which the devices in the standby path arestopped cannot be used. In the mission-critical computer systems, ifcommunication via the operational path cannot be performed, it isrequired that the communication be immediately continued via the standbypath, and thus, a start time of several seconds to several tens ofseconds is unacceptable.

Regarding power saving control for the standby path, Japanese UnexaminedPatent Application Publication No. 2010-198353 discloses a computersystem that includes a power supply control section for controllingpower supply to HBAs connected to a plurality of paths. In the computersystem, when the occurrence of an error with respect to a reissued I/Ois detected, the standby path and the currently-being-used path areswitched, and when a notification of time-out is received, the powersupply control section stops power supply to the HBA in the standbypath.

Furthermore, Japanese Unexamined Patent Application Publication No.2009-289193 discloses a technology of reducing the power consumption ofa device connected to a PCI/PCIe bus, by using the PCI PM (powermanagement) function.

Furthermore, PCI Bus Power Management Interface Specification Rev. 1.2Mar. 3, 2004 prescribes a PM function (power management function)serving as a power saving function for PCI/PCIe-bus expansion cards.

In the technology of Japanese Unexamined Patent Application PublicationNo. 2010-198353, the power supply control section, which controls powersupply to the HBAs, performs power saving control for the standby path.However, many of computer systems that have been put into practical usedo not include this type of power supply control section, so that it isdifficult to immediately apply this technology thereto. In particular,Japanese Unexamined Patent Application Publication No. 2010-198353 doesnot suggest how power saving control for the standby path in a computersystem that uses PCI/PCIe-bus expansion cards having the PCI PM functionis performed.

Furthermore, Japanese Unexamined Patent Application Publication No.2009-289193 proposes the technology of reducing the power consumption ofa device connected to the PCI/PCIe bus, by using the PCI PM function.However, it does not suggest how the technology is applied to a computersystem in which an HBA connected to the PCI/PCIe bus is used maderedundant.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve power saving for astandby path in a computer system in which multiplexed I/O paths aremade for an I/O device by using PCI/PCIe-bus expansion cards having thePCI PM function.

More specifically, the object thereof is to perform control such thatSCSI communication is immediately started by using the standby path thathas been in a power saving state when a failure occurs in theoperational path.

According to a first aspect, the present invention provides a computersystem that performs data communication with an I/O device via host busadaptors (HBAs) connected to the I/O device, the computer systemincluding: as the HBAs, a first HBA that constitutes an operational pathand on which a first PCI/PCIe-bus expansion card having a PCI PMfunction serving as a power saving function is mounted, and a second HBAthat constitutes a standby path and on which a second PCI/PCIe-busexpansion card having the PCI PM function is mounted; and a controlsection that controls the first HBA and the second HBA, in which thecontrol section first performs control such that the first PCI/PCIe-busexpansion card of the first HBA is set to an active state and the secondPCI/PCIe-bus expansion card of the second HBA is set to a power savingstate; when a failure occurs in the operational path, the controlsection performs control such that communication recovery processing forthe operational path is applied to the first HBA and further the secondPCI/PCIe-bus expansion card of the second HBA is changed from the powersaving state to an active state; and, when the communication recoveryprocessing ends unsuccessfully, the control section performs controlsuch that communication is made with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA.

In the above-described computer system, it is preferred that a hostcomputer that performs data communication with the I/O device by using aSCSI command be further included, and the control section concurrentlyperform the control of the communication recovery processing for theoperational path and the control for changing the state of the standbypath from the power saving state to the active state.

Furthermore, in the above-described computer system, it is preferredthat, when a failure in the operational path is detected, the controlsection issue a task management request in the communication recoveryprocessing for the operational path and activate the second HBA in thestandby path, concurrently; and, when the task management request withrespect to the operational path ends unsuccessfully, the control sectionperform communication with the I/O device via the second HBA by using aSCSI command.

Furthermore, in the above-described computer system, it is preferredthat the control section set a PM state of the first PCI/PCIe-busexpansion card of the first HBA in the operational path to D0_Active;the control section set a PM state of the second PCI/PCIe-bus expansioncard of the second HBA in the standby path to D3_hot; the controlsection perform communication with the I/O device via the firstPCI/PCIe-bus expansion card of the first HBA in the operational path;when a problem occurs in the communication using the SCSI command viathe operational path, the control section concurrently perform thecontrol of the communication recovery processing for the operationalpath and the control for changing the PM state of the second HBA in thestandby path to D0_Active; and, when the communication recoveryprocessing for the operational path ends unsuccessfully, the controlsection perform communication with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA.

Furthermore, in the above-described computer system, it is preferredthat, when a problem occurs in the communication using the SCSI commandvia the operational path, the control section concurrently perform thecontrol of the communication recovery processing for the operationalpath and the control for changing the PM state of the second HBA in thestandby path to D0_Active; when the communication recovery processingfor the operational path ends successfully, the control section changethe PM state of the second PCI/PCIe-bus expansion card of the second HBAfrom D0_Active to D3_hot; and the control section further performcommunication with I/O device with a SCSI command by using the firstPCI/PCIe-bus expansion card of the first HBA.

According to a second aspect, the present invention provides a host busadaptor (HBA) control method used in a host computer that performs datacommunication with an I/O device by using a SCSI command via host busadaptors (HBAs) connected to the I/O device, the host computerincluding: as the HBAs, a first HBA that constitutes an operational pathand on which a first PCI/PCIe-bus expansion card having a PCI PMfunction serving as a power saving function is mounted, and a second HBAthat constitutes a standby path and on which a second PCI/PCIe-busexpansion card having the PCI PM function is mounted; and a controlsection that controls the first HBA and the second HBA, the methodincluding the steps of: first performing control, with the controlsection, such that the first PCI/PCIe-bus expansion card of the firstHBA is set to an active state and the second PCI/PCIe-bus expansion cardof the second HBA is set to a power saving state; when a failure occursin the operational path, performing control, with the control section,such that communication recovery processing for the operational path isapplied to the first HBA and further the second PCI/PCIe-bus expansioncard of the second HBA is changed from the power saving state to anactive state; and when the communication recovery processing endsunsuccessfully, performing control, with the control section, such thatcommunication is made with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA.

In the above-described host bus adaptor (HBA) control method, it ispreferred that, when a failure in the operational path is detected, thecontrol section issue a task management request in the communicationrecovery processing for the operational path and activate the second HBAin the standby path, concurrently; and, when the task management requestwith respect to the operational path ends unsuccessfully, the controlsection perform communication with the I/O device via the second HBA byusing a SCSI command.

Furthermore, in the above-described host bus adaptor (HBA) controlmethod, it is preferred that the control section set a PM state of thefirst PCI/PCIe-bus expansion card of the first HBA in the operationalpath to D0_Active; the control section set a PM state of the secondPCI/PCIe-bus expansion card of the second HBA in the standby path toD3_hot; the control section perform communication with the I/O devicevia the first PCI/PCIe-bus expansion card of the first HBA in theoperational path; when a problem occurs in the communication using theSCSI command via the operational path, the control section concurrentlyperform the control of the communication recovery processing for theoperational path and the control for changing the PM state of the secondHBA in the standby path to D0_Active; and, when the communicationrecovery processing for the operational path ends unsuccessfully, thecontrol section perform communication with the I/O device by using thesecond PCI/PCIe-bus expansion card of the second HBA.

Furthermore, in the above-described host bus adaptor (HBA) controlmethod, it is preferred that, when a problem occurs in the communicationusing the SCSI command via the operational path, the control sectionconcurrently perform the control of the communication recoveryprocessing for the operational path and the control for changing the PMstate of the second HBA in the standby path to D0_Active; when thecommunication recovery processing for the operational path endssuccessfully, the control section change the PM state of the secondPCI/PCIe-bus expansion card of the second HBA from D0_Active to D3_hot;and the control section further perform communication with I/O devicewith a SCSI command by using the first PCI/PCIe-bus expansion card ofthe first HBA.

Furthermore, in the above-described host bus adaptor (HBA) controlmethod, it is preferred that the PM states of the first and second HBAsbe changed by rewriting registers (PMCSRs) in PCI CFG spaces generatedon memories mounted on the first and second PCI/PCIe-bus expansioncards.

According to a third aspect, the present invention provides a programfunctioning as a host-bus-adaptor driver, executed on a host computerand controlling a first host bus adaptor (HBA) and a second HBA that areconnected to an I/O device, the host computer including the first HBAthat constitutes an operational path and on which a first PCI/PCIe-busexpansion card having a PCI PM function serving as a power savingfunction is mounted, and the second HBA that constitutes a standby pathand on which a second PCI/PCIe-bus expansion card having the PCI PMfunction is mounted, and the host computer performing data communicationwith the I/O device using a SCSI command via the first HBA or the secondHBA, in which the host-bus-adaptor driver first performs control suchthat the first PCI/PCIe-bus expansion card of the first HBA is set to anactive state and the second PCI/PCIe-bus expansion card of the secondHBA is set to a power saving state; when a failure occurs in theoperational path, the host-bus-adaptor driver performs control such thatcommunication recovery processing for the operational path is applied tothe first HBA and further the second PCI/PCIe-bus expansion card of thesecond HBA is changed from the power saving state to an active state;and when the communication recovery processing ends unsuccessfully, thehost-bus-adaptor driver performs control such that communication is madewith the I/O device by using the second PCI/PCIe-bus expansion card ofthe second HBA.

According to the present invention, it is possible to realize powersaving for the standby path by using the PCI PM function, in thecomputer system in which the multiplexed I/O paths are made for the I/Odevice by using PCI/PCIe-bus expansion cards having the PCI PM function.More specifically, it is possible to perform control such that SCSIcommunication is immediately started by using the standby path that hasbeen in the power saving state when a failure occurs in the operationalpath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example configuration of a computersystem according to one embodiment;

FIG. 2 is a diagram showing PCI function power management statetransitions;

FIG. 3 is a diagram showing, in outline, a PCI CFG space;

FIG. 4 is a sequence diagram showing command processing for HBAs and apower control action (control action 1), performed by an HBA driveraccording to the embodiment; and

FIG. 5 is a sequence diagram showing command processing for the HBAs anda power control action (control action 2), performed by the HBA driveraccording to the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will be described belowwith reference to the drawings.

FIG. 1 shows an example configuration of a computer system according tothe embodiment.

A host computer 10 is connected to ports 111 and 112 of a storage system11 that serves as an I/O device by a plurality of host bus adaptors(referred to as HBAs) 106 and 107 via multiplexed (redundant) I/O paths113 and 114. The I/O path 113 serves as an operational path, and the I/Opath 114 serves as a standby path. The host computer 10 can usually usethe HBA 106 (operational path) to access a logical unit 115 generated inthe storage system 11. If a failure occurs in the operational path, thehost computer 10 can use the HBA 107 (standby path) to access thelogical unit 115 in the storage system 11 via the standby I/O path 114and the port 112. In this embodiment, each of the two HBAs 106 and 107is formed of a PCI/PCIe-bus expansion card with a PCI PM capability. Oneach of the HBAs 106 and 107, the following units (not shown) are mainlymounted: a processor that executes a program to perform data transfercontrol; an LSI having a memory for storing various data and the programfor the control; firmware that performs the entire control of thecorresponding HBA; and an optical module that transmits and receivesdata to and from the corresponding port. In the figure, referencenumerals 108 and 109 denote PM states.

Although a hardware configuration of the host computer 10 is not shownin the figure, the host computer 10 includes a processor that activatesan OS (operating system) and also executes an application program 101,and a memory that stores various programs and data. The host computer 10uses a host bus adaptor (HBA) driver 102.

The HBA driver 102 is a program executed by the processor to realizefunctions and has, as its inner functions, an adaptor driver commonsection 103, an adaptor control instance 104, and an adaptor controlinstance 105. The adaptor control instance 104 controls the HBA 106, andthe adaptor control instance 105 controls the HBA 107. The adaptordriver common section 103 controls each of the adaptor control instances104 and 105 independently to control data communication and alsoperforms control such that the adaptor control instance 104 in theoperational path is switched to the adaptor control instance 105 in thestandby path.

In this embodiment, data communication is performed between the hostcomputer 10 and the I/O device according to a SCSI standard, forexample, based on a SCSI architecture model 4 described in Working DraftAmerican National Standard Project T10/1683-D Rev. 6, 23 May 2006. Whenthe application program 101 performs SCSI data communication with thelogical unit 115 of the storage system 11, the adaptor driver commonsection 103 in the HBA driver 102 transmits a SCSI command via the I/Opath 113, which passes through the HBA 106. Unless no response (that is,a data communication failure) occurs in the HBA 106 in the operationalpath, SCSI communication through the HBA 107 in the standby path is notperformed.

FIG. 2 shows PCI function power management state transitions accordingto page 45 in PCI Bus Power Management Interface Specification Rev. 1.2Mar. 3, 2004.

In the embodiment of the present invention, power saving control for thestandby path is performed while transition occurs among three states,i.e., DO Unitialized, DO Active, and D3_hot, shown in this transitiondiagram. In the D3-hot state, power is supplied, for example, to aconnection bus circuit and a power management circuit in the HBA, amicroprocessor and the LSI having the memory are in the stopped state,and power can be stopped thereto.

FIG. 3 is a diagram showing, in outline, a PCI CFG space. Note that thisis described in pages 21 to 24 in PCI Bus Power Management InterfaceSpecification Rev. 1.2 Mar. 3, 2004.

The PCI CFG space is generated in the memory of each of the HBAs 106 and107. A PCI CFG space 300 of each of the HBAs 106 and 107 includes apower management capability ID 301 and a power management control/statusregister (PMCSR) 302. The PMCSR 302 stores power-status bit information,and one of the three states, i.e., DO Unitialized, DO Active, andD3_hot, can be selected by rewriting the power-status bit information.Specifically, the adaptor control instance 104 rewrites the PMCSR 302 ofthe HBA 106 to switch the PCI PM state 108 of the HBA 106. Similarly,the adaptor control instance 105 rewrites the PMCSR 302 of the HBA 107to switch the PCI PM state 109 thereof. The PCI PM state transitions inthis embodiment are shown in FIG. 2.

When the host computer 10 is activated, control is performed such thatthe adaptor control instance 104 sets the PCI PM state 108 of the HBA106 to “D0_Active”, and the adaptor control instance 105 sets the PCI PMstate 109 of the HBA 107 to “D3_hot”. Specifically, “b11” is written inthe PMCSR 302 of the HBA 107 to set the PCI PM state 109 to “D3_hot”,thus causing the HBA 107 to enter a warm standby state. At this time,some of the functions are stopped in the HBA 107, thus reducing thepower consumption, compared with the HBA 106 set in the “D0_Active”state. Specifically, in the HBAs 106 and 107 of this embodiment, whenthe PCI PM state 109 is set to “D3_hot”, the function is stopped, theoptical module for communication is turned off, a DMA transfer functionis stopped, and a frame transmission and reception function is stopped.Thus, power saving can be achieved.

Next, command processing for the HBAs and power control actionsperformed by the HBA driver will be described with reference to FIGS. 4and 5.

In the initial state, the PM state 108 of the HBA 106 is set to “DOActive”, and the PM state 109 of the HBA 107 is set to “D3 hot”. It isassumed that the corresponding power-status bit information isregistered in that way in the PMCSR 302 of the PCI CFG space 300.Furthermore, it is assumed that the adaptor control instance 104 for theoperational path has a software timer used to monitor the time(time-out) of a response to a command or a request issued to the HBA106.

In FIG. 4, it is assumed that SCSI communication is attempted from theapplication program 101 to the logical unit 115 of the storage system11. At this time, the adaptor driver common section 103 of the HBAdriver 102 issues a SCSI command transmission request to the adaptorcontrol instance 104 (S400).

When the SCSI command transmission request is received, the adaptorcontrol instance 104 transmits a SCSI command to the HBA 106 (S401). Itis assumed that, after the SCSI command is transmitted, no response isreceived from the HBA 106 (that is, a failure occurs therein) (S402). Inthis case, the adaptor control instance 104 detects the time-out of aSCSI-command response waiting timer (S403) and notifies the fact to theadaptor driver common section 103.

When the time-out of the SCSI-command response waiting timer is detected(S403), the adaptor control instance 104 issues a task managementrequest to the HBA 106 in order to recover communication between the HBA106 and the logical unit 115 (S407). Note that a task management requestand processing related thereto in the SCSI architecture are described inSCSI Architecture Model 4 (pages 24 to 25 and 94 in Working DraftAmerican National Standard Project T10/1683-D Rev. 6, 23 May 2006).

After the task management request is issued, if no response is receivedfrom the HBA 106 (S408), the adaptor control instance 104 detects thetime-out of a task-management response waiting timer and notifies thefact to the adaptor driver common section 103 (S410).

When the adaptor driver common section 103 receives the notification ofthe time-out of the SCSI-command response waiting timer from the adaptorcontrol instance 104 (S403), the adaptor driver common section 103issues an adaptor initializing request to the adaptor control instance105 (S404). When the adaptor initializing request is received, theadaptor control instance 105 writes power-status bit information “b00”in the PMCSR 302 of the HBA 107 in order to prepare for SCSIcommunication via the HBA 107. Then, the adaptor control instance 105issues a PM state change to the HBA 107 (S406) to change the PCI PMstate 109 of the HBA 107 to “D0_Uninitialized” (S421). Then, the adaptorcontrol instance 105 initializes the HBA 107 (S411), and the HBA 107enters the “D0_Active” state (S422). In this way, after thosepreparation processes (S406 and S411), the HBA 107 is capable ofperforming SCSI communication with the logical unit 115.

When the notification of the time-out of the task-management responsewaiting timer is received from the adaptor control instance 104 (S410)(that is, when the operational path, which includes the HBA 106, goesdown), the adaptor driver common section 103 issues a SCSI commandtransmission request to the adaptor control instance 105 (S412). Whenthe SCSI command transmission request is received (S412), the adaptorcontrol instance 105 transmits a SCSI command to the HBA 107 (S413).Specifically, the SCSI command causes the HBA 107 in the standby path toaccess the logical unit 115 of the storage system 11.

According to this embodiment, if a response to the SCSI command (S401)issued by the adaptor control instance 104 is not returned from the HBA106 (S403), the adaptor driver common section 103 immediately issues theadaptor initializing request to the adaptor control instance 105 (S404)to instruct it to change the PCI PM state of the HBA 107 to“D0_Uninitialized” (S421). Furthermore, the task-management-relatedprocesses (S407 and S410) performed by the adaptor control instance 104and the preparations (S406 and S411) performed by the adaptor controlinstance 105 are concurrently carried out (during the period of S405surrounded by a dotted line), thereby enabling the HBA 107 to performSCSI communication (S412 and S413) with the storage system 11immediately after the adaptor control instance 104 detects the time-outof the task-management response waiting timer (S410). Through thiscontrol action, SCSI communication via the standby path, which includesthe HBA 107, can be immediately started.

The above-described example shown in FIG. 4 is a control action carriedout when a task-management time-out response to the task managementrequest issued by the adaptor control instance 104 is returned (S410)because a failure occurs in the HBA 106 (S408).

In contrast, an example shown in FIG. 5 is a control action carried outwhen a failure in the HBA 106 is not detected, and the task managementwith respect to the task management request issued by the adaptorcontrol instance 104 ends successfully.

In FIG. 5, when a task management response to the task managementrequest is received (S510), the adaptor control instance 104 notifies atask management success to the adaptor driver common section 103 (S511).

When the task management success is received, the adaptor driver commonsection 103 issues an adaptor standby request to the adaptor controlinstance 105 (S512). Then, the adaptor driver common section 103attempts SCSI communication with the HBA 106 set in the normal state.Specifically, the adaptor driver common section 103 issues a SCSIcommand transmission request to the adaptor control instance 104 (S514),and the adaptor control instance 104 transmits a SCSI command to the HBA106 (S515).

When the adaptor standby request is received, the adaptor controlinstance 105 changes the PM state of the HBA 107 (S513). Specifically,the adaptor control instance 105 writes “b11” in the PMCSR 302 of theHBA 107 to set the PCI PM state to “D3_hot” (109) (S423), thereby makingthe HBA 107 enter the standby state.

As described above, according to this embodiment, when the adaptorcontrol instance 104 receives the task management response to the taskmanagement request, the HBA 107 can be set in the warm standby stateagain. Thus, power saving can be continued.

What is claimed is:
 1. A computer system that performs datacommunication with an I/O device via host bus adaptors (HBAs) connectedto the I/O device, the computer system comprising: a first HBA thatconstitutes an operational path and on which a first PCI/PCIe-busexpansion card having a PCI PM function serving as a power savingfunction is mounted, and a second HBA that constitutes a standby pathand on which a second PCI/PCIe-bus expansion card having the PCI PMfunction is mounted; a control section that is configured to control thefirst HBA and the second HBA, wherein the control section is configuredto first performs control such that the first PCI/PCIe-bus expansioncard of the first HBA is set to an active state and the secondPCI/PCIe-bus expansion card of the second HBA is set to a power savingstate; wherein, when a failure occurs in the operational path, thecontrol section is configured to: perform a communication recoveryprocess for the operational path applied to the first HBA, wherein thecommunication recovery process comprises: sending a task managementrequest to the first HBA and receiving the response of the taskmanagement request from the first HBA, and further perform changing thesecond PCI/PCIe-bus expansion card from the power saving state to anactive state; and when the communication recovery processing endsunsuccessfully, the control section is configured to perform controlsuch that communication is made with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA, wherein the controlsection is configured to perform the changing before receiving theresponse of the task management request.
 2. The computer systemaccording to claim 1, further comprising a host computer that performsdata communication with the I/O device by using a SCSI command; whereinthe control section is configured to concurrently perform the control ofthe communication recovery processing for the operational path and thecontrol for changing the state of the standby path from the power savingstate to the active state.
 3. The computer system according to claim 1,wherein, when a failure in the operational path is detected, the controlsection is configured to issue a task management request in thecommunication recovery processing for the operational path and activatesthe second HBA in the standby path, concurrently; and when the taskmanagement request with respect to the operational path endsunsuccessfully, the control section is configured to performcommunication with the I/O device via the second HBA by using a SCSIcommand.
 4. The computer system according to claim 2, wherein thecontrol section is configured to set a PM state of the firstPCI/PCIe-bus expansion card of the first HBA in the operational path toD0_Active; the control section is configured to set a PM state of thesecond PCI/PCIe-bus expansion card of the second HBA in the standby pathto D3_hot; the control section is configured to perform communicationwith the I/0 device via the first PCI/PCIe-bus expansion card of thefirst HBA in the operational path; when a problem occurs in thecommunication using the SCSI command via the operational path, thecontrol section is configured to concurrently perform the control of thecommunication recovery processing for the operational path and thecontrol for changing the PM state of the second HBA in the standby pathto D0_Active; and when the communication recovery processing for theoperational path ends unsuccessfully, the control section is configuredto perform communication with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA.
 5. The computer systemaccording to claim 4, wherein, when a problem occurs in thecommunication using the SCSI command via the operational path, thecontrol section is configured to concurrently perform the control of thecommunication recovery processing for the operational path and thecontrol for changing the PM state of the second HBA in the standby pathto D0_Active; when the communication recovery processing for theoperational path ends successfully, the control section is configured tochange the PM state of the second PCI/PCIe-bus expansion card of thesecond HBA from D0_Active to D3_hot; and the control section isconfigured to further perform communication with I/O device with a SCSIcommand by using the first PCI/PCIe-bus expansion card of the first HBA.6. A host bus adaptor (HBA) control method used in a host computer thatperforms data communication with an I/O device by using a SCSI commandvia host bus adaptors (HBAs) connected to the I/O device, the hostcomputer including: as the HBAs, a first HBA that constitutes anoperational path and on which a first PCI/PCIe-bus expansion card havinga PCI PM function serving as a power saving function is mounted, and asecond HBA that constitutes a standby path and on which a secondPCI/PCIe-bus expansion card having the PCI PM function is mounted; and acontrol section that is configured to control the first HBA and thesecond HBA, the method comprising the steps of: first performingcontrol, with the control section, such that the first PCI/PCIe-busexpansion card of the first HBA is set to an active state and the secondPCI/PCIe-bus expansion card of the second HBA is set to a power savingstate; wherein, when a failure occurs in the operational path,performing control, with the control section, using a communicationrecovery process for the operational path applied to the first HBA,wherein the communication recovery process comprises: sending a taskmanagement request to the first HBA and receiving the response of thetask management request from the first HBA, and further performingchanging the second PCI/PCIe-bus expansion card from the power savingstate to an active state; and when the communication recovery processingends unsuccessfully, performing control, with the control section, suchthat communication is made with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA, wherein the controlsection is configured to perform the changing before receiving theresponse of the task management request.
 7. The host bus adaptor (HBA)control method according to claim 6, wherein, when a failure in theoperational path is detected, issuing by the control section a taskmanagement request in the communication recovery processing for theoperational path and activating the second HBA in the standby path,concurrently; and when the task management request with respect to theoperational path ends unsuccessfully, the control section performingcommunication with the I/O device via the second HBA by using a SCSIcommand.
 8. The host bus adaptor (HBA) control method according to claim6, setting by the control section a PM state of the first PCI/PCIe-busexpansion card of the first HBA in the operational path to D0_Active;setting by the control section a PM state of the second PCI/PCIe-busexpansion card of the second HBA in the standby path to D3_hot;performing by the control section communication with the I/O device viathe first PCI/PCIe-bus expansion card of the first HBA in theoperational path; when a problem occurs in the communication using theSCSI command via the operational path, currently performing by thecontrol section the control of the communication recovery process forthe operational path and the control for changing the PM state of thesecond HBA in the standby path to D0_Active; and when the communicationrecovery processing for the operational path ends unsuccessfully,performing by the control section communication with the I/O device byusing the second PCI/PCIe-bus expansion card of the second HBA.
 9. Thehost bus adaptor (HBA) control method according to claim 8, wherein,when a problem occurs in the communication using the SCSI command viathe operational path, concurrently performing by the control section thecontrol of the communication recovery processing for the operationalpath and the control for changing the PM state of the second HBA in thestandby path to D0_Active; when the communication recovery processingfor the operational path ends successfully, changing by the controlsection the PM state of the second PCI/PCIe-bus expansion card of thesecond HBA from D0_Active to D3_hot; and further performing by thecontrol section communication with I/O device with a SCSI command byusing the first PCI/PCIe-bus expansion card of the first HBA.
 10. Thehost bus adaptor (HBA) control method according to claim 8, wherein thePM states of the first and second HBAs are changed by rewritingregisters (PMCSRs) in PCI CFG spaces generated on memories mounted onthe first and second PCI/PCIe-bus expansion cards.
 11. A non-transitorycomputer readable medium storing a computer-readable program functioningas a host-bus-adaptor driver, executed on a host computer andcontrolling a first host bus adaptor (HBA) and a second HBA that areconnected to an I/O device, the host computer including the first HBAthat constitutes an operational path and on which a first PCI/PCIe-busexpansion card having a PCI PM function serving as a power savingfunction is mounted, and the second HBA that constitutes a standby pathand on which a second PCI/PCIe-bus expansion card having the PCI PMfunction is mounted, and the host computer performing data communicationwith the I/O device using a SCSI command via the first HBA or the secondHBA, comprising: computer-readable instructions to first perform by thehost-bus-adaptor driver control such that the first PCI/PCIe-busexpansion card of the first HBA is set to an active state and the secondPCI/PCIe-bus expansion card of the second HBA is set to a power savingstate; when a failure occurs in the operational path, computer readableinstructions to perform by the host-bus-adaptor driver control, usingcommunication recovery process for the operational path is applied tothe first HBA, wherein the communication recovery process comprises:sending a task management request to the first HBA and receiving theresponse of the task management request from the first HBA, and computerreadable instructions to change the second PCI/PCIe-bus expansion cardof the second HBA from the power saving state to an active state beforereceiving the response of the task management request; and when thecommunication recovery processing ends unsuccessfully, computer readableinstructions to perform by the host-bus-adaptor driver control such thatcommunication is made with the I/O device by using the secondPCI/PCIe-bus expansion card of the second HBA.